On 9/16/2012 8:04 AM, Gerrit Heitsch wrote: > On 09/16/2012 01:31 PM, firstname.lastname@example.org wrote: >> >> On 2012-09-16, at 12:57, Gerrit Heitsch wrote: >> >>> The simplest way to achieve that is a 74LS257. >> >> Yes, or 157.. as we don't need the cutoff of the output anyway. > > Yes, but the advantage of the 74LS257 is that if you do hardware work > on the C64, C16, +4... you will have some 74LS257 at home. > > The LS157 doesn't have tristate outputs, but still has an enable > input, so no matter how you slice it, it's a signal you need to take > care of. > > >> Yup, levels are very much my worry here but I wanted to avoid adding >> another IC, especially one with most pins unused. > > After thinking about it for a bit... You can also use a 74LS00 (or > 74HCT00) which is fully used up in the process and has 2 pins less: > > How to set up: > > Input1 is A12, it gets fed into one input of one NAND. > > Input2 is your switch. It gets fed into one input of another NAND. > Remember the pullup here. > > The select signal (_Char-ROM) is directly fed into the other input of > the NAND with A12. It is also inverted by using one NAND (second input > tied HIGH) and then fed into the second input of the NAND that takes > your switch input. > > The outputs of the 2 NANDs are then fed into the last available NAND > and its output is A12 for the EPROM. > > Should look like this: > > http://ecee.colorado.edu/~ecen3100/lab2_files/2-1_NAND_mux.PNG > > Gerrit > > > > Message was sent through the cbm-hackers mailing list Looking at the original schematic, I sketched one up using a '148 and a '257. With the '257 as Gerrit suggested (the NAND idea would also work) and the 148 handling the 3 signals. The '148 takes care of OE/CE with it's GS pin (14), and the design requires just 2 ICs and 1 resistor (to hold the CHARROM switch position in high unless shorted through the switch/jumper. Jim -- Jim Brain email@example.com www.jbrain.com Message was sent through the cbm-hackers mailing listReceived on 2012-09-17 05:00:04
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